1. Field of the Invention
The present invention relates to a wavelength-division-multiplexer/demultiplexer, and more particularly to a wavelength-division-multiplexer/demultiplexer comprising an arrayed waveguide grating.
2. Description of the Related Art
Generally, in a wavelength-division-multiplexed (WDM) optical communication network, optical signals including multiple channels are transmitted simultaneously via a single strand of an optical fiber. In the WDM network based on a single-mode optical fiber, an Arrayed Waveguide Grating (AWG) using a planar lightwave circuit (PLC) is used as a wavelength-division-multiplexer/demultiplexer.
In such a wavelength-division-multiplexer/demultiplexer incorporating the arrayed waveguide grating, preferably, each of the wavelength bands of divided channels must not deviate from its center wavelength. However, the wavelength bands in an actual application may deviate from their center wavelengths due to various external environmental factors or internal factors of the system. Currently, this problem is resolved by applying a parabolic horn-type structure to an input waveguide. That is, the use of the parabolic horn-type input waveguide serves to form a flattened waveform of an outputted optical signal.
FIG. 1 is a perspective view of a conventional wavelength-division-multiplexer/demultiplexer. As shown in FIG. 1, the wavelength-division-multiplexer/demultiplexer 100 comprises an input waveguide 113, an arrayed waveguide grating 111, a first slab waveguide 117, output waveguides 115, and a second slab waveguide 119. These optical components are provided on a planar lightwave circuit (PLC) 101.
The input waveguide 113 inputs a multiplexed optical signal to the wavelength-division-multiplexer/demultiplexer 100. However, in the case that the wavelength-division-multiplexer/demultiplexer 100 is operated as a multiplexer, the input waveguide 113 outputs the multiplexed optical signal.
FIG. 2 shows a partially-enlarged view of the input waveguide 113 as shown in FIG. 1. As shown, the input waveguide 113 has a parabolic horn shape, in which its width is gradually increased in a progressing direction of an optical signal. A variation of the width of the input waveguide 113 is determined by Equation 1, shown below, and the shape of the input waveguide 113 is defined by Equation 2, also shown.                                                         1              2                        ⁢                                          ⅆ                                  W                  1                                                            ⅆ                z                                              =                                    -              γ                        ⁢                                          4                ⁢                                                                   ⁢                π                            3                        ⁢                                          λ                g                                            W                                  i                  ⁢                                                                           ⁢                  l                                                                    ⁢                                                       [                  Equation          ⁢                                           ⁢          1                ]                                W        =                                                            (                                                      2                    ⁢                                                                                   ⁢                    α                    ⁢                                                                                   ⁢                                          λ                      g                                        ⁢                    z                                    +                                      W                    0                    2                                                  )                                            1                2                                      ⁢            α                    =                      -                                          8                ⁢                                                                   ⁢                π                ⁢                                                                   ⁢                γ                            3                                                          [                  Equation          ⁢                                           ⁢          2                ]            (W0: a width of the parabolic horn-type waveguide at a starting position of an optical signal, α: a coupling coefficient, λg: an effective wavelength, z: a length of the parabolic horn-type waveguide, and W: a width of the parabolic horn-type waveguide at a certain position.)
Herein, z1 denotes a measured distance from an input terminal of the input waveguide 113 to a position of the progressing optical signal; W1 is a width of the input waveguide 113 at the position the progressing optical signal; Wi1 is a width of the input waveguide 113 at the input terminal; λg denotes an effective wavelength of the optical signal, and α denotes a coupling coefficient between the fundamental and higher modes of the optical signal.
When the multiplexed optical signal progresses along the parabolic horn-type input waveguide 113, the mode coupling of the optical signal from the fundamental mode to the secondary mode or the higher mode occurs, and the bandwidth of the optical signal is expanded. In addition, due to the mode coupling of the optical signal from the fundamental mode to the higher mode, the optical signal exhibits a flattened field distribution. Equation 3 illustrates a power (Pj) distribution of the coupled optical signal from the fundamental mode to the higher mode as the multiplexed optical signal progresses along the input waveguide 113. Equation 4 illustrates a relationship between the coupling coefficient created by the optical signal progressing along the input waveguide 113 and the maximum value of the power (Pjmax) of the optical signal coupled from the fundamental mode to the higher mode. Further, FIG. 3 is a graph illustrating the power (Pj) distribution of the coupled optical signal from the fundamental mode to the higher mode as the multiplexed optical signal progresses along the input waveguide 113.                               α          j                =                              α                          j              ⁢                                                           ⁢              0                                ⁢                                    2              ⁢                                                           ⁢              γ                                                      (                                                      4                    ⁢                                          γ                      2                                                        +                  1                                )                                            1                2                                              ⁢                      ⅇ                                          j                ⁢                                                                   ⁢                u                            2                                ⁢                      sin            ⁡                          [                                                1                  2                                ⁢                                                      (                                                                  4                        ⁢                                                                                                   ⁢                                                  γ                          2                                                                    +                      1                                        )                                                        1                    2                                                  ⁢                u                            ]                                                          [                  Equation          ⁢                                           ⁢          3                ]                                                                    P                              j                ⁢                                                                   ⁢                max                                                    P                              j                ⁢                                                                   ⁢                0                                              =                                                                      4                  ⁢                                                                           ⁢                                      γ                    2                                                                                        4                    ⁢                                                                                   ⁢                                          γ                      2                                                        +                  1                                            ⁢              γ                        =                                          3                ⁢                α                                            4                ⁢                                                                   ⁢                π                                                    ⁢                                                       [                  Equation          ⁢                                           ⁢          4                ]            
In the above Equations 3 and 4, Pjmax denotes the power of an optical signal coupled from the fundamental mode to the higher mode; and Pj0 denotes the power of an optical signal initially inputted to the input waveguide 113. As shown in the above Equations 3 and 4, the power (Pj) of the optical signal coupled from the fundamental mode to the higher mode varies according to the width (Wi1) of the input waveguide 113 at the input terminal and the width (W1) of the input waveguide 313 at a certain position. FIG. 3 illustrates a variation of the power (Pj) of the optical signal coupled from the fundamental mode into the higher mode according to a variation of the width of the waveguide. The power (Pjmax) of the optical signal coupled from the fundamental mode into the higher mode defines a transition power of the optical signal, and the power (Pj0) of the optical signal initially inputted to the input waveguide 113 defines an input power.
The first slab waveguide 117 is formed between the input waveguide 113 and the arrayed waveguide grating 111, and it serves to branch the optical signal inputted via the input waveguide 113 and then input the branched signals to the arrayed waveguide grating 111.
The arrayed waveguide grating 111 comprises a plurality of phase-modulation waveguides with different paths. That is, each of the waveguides of the arrayed waveguide grating 111 has a designated path differing from those of the neighboring waveguides.
The optical signal flattened by the parabolic horn-type input waveguide 113 passes through the arrayed waveguide grating 111, then forms as demultiplexed multiple channels on an output surface of the second slab waveguide 119. Each of the multiple channels formed on the output surface of the second slab waveguide 119 has a flattened waveform by the coupling between the fundamental and higher modes within the parabolic horn-type input waveguide 113, and then it is outputted as an independent channel with a center wavelength via the output waveguide 115.
However, the output waveguide 115 does not pass the higher modes of the optical signal flattened by the input waveguide 113 except for the fundamental mode of the optical signal. This causes a loss of the power of the optical signal of the wavelength-division-multiplexer/demultiplexer.